Polyolefins, especially polyethylenes, are important thermoplastics with many uses. Traditionally, polyolefins have been manufactured using slurry, solution, and gas-phase polymerization processes and Ziegler-Natta catalysts. In recent years, polyolefins made using single-site catalysts, including metallocenes, have been commercialized.
Polyolefin customers continue to seek resins that have both high stiffness and good environmental stress crack resistance (ESCR), especially for molding and extrusion applications. Unfortunately, for resins made with similar catalyst and process technologies, at a given melt index, resins with higher stiffness usually have poorer ESCR.
The molecular weight distribution of a polymer resin influences its processability and physical properties. The most common measure of molecular weight distribution is Mw/Mn, the ratio of weight average to number average molecular weight, which is usually determined by gel permeation chromatography (GPC). Resin properties and processability are also influenced by long-chain branching and comonomer distribution, and this information is not readily ascertained from GPC results.
Valuable information about polydispersity that takes into account differences in long-chain branching is available from rheological measurements on molten resins, i.e., from “rheological polydispersity.” An overall polydispersity ratio (PDR) that uses complex viscosity data as a function of complex modulus rather than frequency can be measured. An additional measure of rheological polydispersity is ER, which is determined from plots of storage modulus (G′) versus loss modulus (G″) and is a measure of high-molecular-weight-end polydispersity. Both PDR and ER are conveniently determined as discussed in R. Shroff and H. Mavridis, New Measures of Polydispersity from Rheological Data on Polymer Melts, J. Appl. Polym. Sci. 57 (1995) 1605. In spite of the availability of this tool, differences in rheological polydispersity have not often been exploited. For a notable exception, however, see U.S. Pat. No. 6,713,585, in which ER measurements and their shifts upon pelletization were shown to be important for identifying and characterizing new ethylene copolymer resins.
Colin Li Pi Shan et al. studied the rheological properties of HDPE/LLDPE reactor blends having bimodal microstructures (see Polymer 44 (2003) 177). Some of the blends disclosed have a relatively high-molecular-weight (HMW), low-density copolymer component and a relatively low-molecular-weight (LMW), high-density polyethylene homopolymer component (see Table 1). FIG. 6 of the reference shows three rheology plots of G′ versus G″. From this data, it is apparent that the rheological polydispersity of the LMW component cannot be greater than that of either the HMW component or that of a blend of the two components. As is explained in Shroff and Mavridis, supra, at page 1621 and FIG. 2, “at a given level of G”, the broader the spectrum, the higher the G′.” A “broader spectrum” is synonymous with a higher rheological polydispersity. In Li Pi Shan's FIG. 6 plots, the resin component with relatively low G′ values therefore has a relatively low rheological polydispersity. In the top two plots, which show a HMW low-density copolymer, a LMW high-density homopolymer, and blends of the two, the LMW homopolymer has a rheological polydispersity less than that of either the HMW copolymer or a blend of the HMW and LMW components.
Commercial polyolefin resins are commonly produced in a single reactor with a Ziegler-Natta catalyst, ethylene, enough hydrogen to control molecular weight, and enough comonomer to drive density down to a targeted value. The resulting resins have more of the comonomer typically incorporated into the lower molecular weight component of the resin. Dual reactor configurations are also used. Even with two parallel reactors, however, manufacturers often choose to make products of similar density and melt index in each reactor and then blend them to give resins with narrow molecular weight distribution that are valuable for injection molding (see Comparative Examples 2, 4, and 6 below). Usually, these resins lack optimum ESCR.
Still needed in the industry are resins that have a desirable balance of high stiffness and good ESCR. Particularly needed is a better appreciation of how differences in rheological polydispersity might be exploited to arrive at the improved resins. Methods for making the resins using readily available catalysts, equipment, and processes are also needed.